qt: synchronize the color schemes with Qt 6.8.0

Qt finally provides the ability to change the color scheme. Although the default palette colors are rarely used in Qt Quick, this is still an important thing for the platform plugin.

This means that if the user opts in to a specific scheme, things such as platform menus, windows, window frames would adapt to the overridden scheme rather than using the setting inherited from the system.

Request review @chub.